1. Field of the Invention
The present invention relates to a rechargeable lithium battery, and more particularly to a rechargeable lithium battery which utilizes the improved active material for its positive or negative electrode.
2. Description of Related Art
In recent years, rechargeable lithium batteries have been extensively developed. The performance characteristics of rechargeable batteries, such as charge-discharge voltages, charge-discharge cycle life characteristics and storage capabilities, depend largely on the particular electrode active material used. This has led to the search of various active materials.
In Japanese Patent Laid Open No. Hei 6-275263 (1994), a rechargeable lithium battery is disclosed which uses an Lixe2x80x94Ti complex oxide represented by the formula LixTiyO4. Such a rechargeable lithium battery utilizing the Lixe2x80x94Ti complex oxide for active material has been used as a power source for electronic devices, such as watches. Among various Lixe2x80x94Ti complex oxides, Li4Ti5O12 having a spinel structure has been reported as allowing a larger amount of lithium to be electrochemically reversibly inserted thereinto and extracted therefrom, with little modification of its crystal structure during repeated lithium. insertion-extraction cycles, so that it undergoes less deterioration during charge-discharge cycles (See, for example, T. Ohzuku, J. Electrochem. Soc., 142, p.1431, 1995). However, this active material has also been reported as being a compound having an extremely low electron conductivity (See, for example, K. M. Colbow, J. Power Sources, 26, p.397, 1989). This property has imposed problems, such as creating a high reaction resistance during lithium insertion and extraction or resulting in a marked reduction of characteristics at heavy loads, making it difficult for the active material to be applied to battery systems requiring discharge at heavy loads. The use of such an active material, in the form of fine particles, has been recently contemplated to improve battery characteristics (See, for example, D. Peramunage, J. Electrochem. Soc., 145, p.2609, 1998).
However, the attempt of simply subdividing particles has been still insufficient to achieve improvements of discharge characteristics at heavy loads.
The present invention is directed toward solving the above-described conventional problems and its object is to provide a rechargeable lithium battery which has imparted thereto good discharge load characteristics by utilizing the Lixe2x80x94Ti complex oxide as active material.
The rechargeable lithium battery of the present invention has a positive electrode, a negative electrode and a non-aqueous electrolyte. Characteristically, the positive or negative electrode contains, as its active material, an Lixe2x80x94Ti complex oxide represented by the compositional formula Li4MxTi5xe2x88x92xO12 and having a spinel crystal structure, wherein M is selected from at least one of V, Nb, Mo and P, and x satisfies the relationship 0 less than xxe2x89xa60.45.
In accordance with the present invention, the inclusion of the element M (at least one of V, Nb, Mo and P) in the crystal lattice of Lixe2x80x94Ti complex oxide results in the reduced reaction resistance when lithium is inserted into or extracted from the active material. The use of the above-specified Lixe2x80x94Ti complex oxide for active material of the positive or negative electrode thus leads to the improvement in load characteristics of the rechargeable lithium battery.
In the present invention, those elements which, when introduced into Li4Ti5O12, showed appreciable contribution to the improvement in load characteristics are specified herein as the elements M. Ti in the composition Li4Ti5O12 is present in the tetravalent cation form. The element M, as specified in the present invention, is the element which exists in a stable pentavalent or hexavalent cation form in an oxide thereof. It is thus believed that the introduction of the element M into the composition Li4Ti5O12 increases a density of electrons which contribute to the electron conductivity in the crystal to result in the increased electron conductivity of the resulting complex oxide, which effectively decreases the reaction resistance.
In the present invention, the stoichiometry x of the element M in the above-specified compositional formula is maintained not to exceed 0.45. If the inclusion of the element M exceeds 0.45, an oxide phase of M only may be produced to result in lowering the improving effect of load characteristics.
In the present invention, the aforementioned Lixe2x80x94Ti complex oxide for use as the positive or negative active material has a spinel crystal structure. The presence of such a crystal structure can be identified by X-ray diffraction (XRD).
An electrolyte solvent for use in the rechargeable lithium battery according to the present invention can be selected from non-aqueous electrolyte solvents generally employed for rechargeable lithium batteries. Specifically, it may be a mixed solvent of cyclic carbonate and chain carbonate, for example. Examples of cyclic carbonates include ethylene carbonate, propylene carbonate and butylene carbonate. Examples of chain carbonates include dimethyl carbonate, methyl ethyl carbonate and diethyl carbonate. The electrolyte solvent may alternatively be a mixed solvent of the aforestated cyclic carbonate and an ether solvent, for example. Examples of ether solvents include 1,2-dimethoxyethane, 1,2-diethoxyethane and the like. Examples of useful electrolyte solutes include LiPF6, LiBF4, LiCF3SO3, LiN(CF3SO2)2, LiN(C2F5SO2)2, LiN(CF3SO2)(C4F9SO2), LiC(CF3SO2)3, LiC(C2F5SO2)3 and any combination thereof, for example. Other applicable electrolytes include gelled polymer electrolytes wherein a liquid electrolyte is impregnated in polymers such as polyethylene oxide and polyacrylonitrile, and inorganic solid electrolytes such as LiI and Li3N, for example.
In the present invention, any non-aqueous electrolyte can be used, so long as it contains an Li compound as a solute for realizing an ionic conductivity, and a solvent used to dissolve and hold the solute is hardly decomposed at voltages during battery charge, discharge and storage.
In the case where the aforementioned Lixe2x80x94Ti complex oxide is used as the positive active material, a suitable negative active material may be chosen from carbon materials capable of electrochemical storage and release of Li, such as graphite (either natural or synthetic), coke, and calcined organics; Li alloys such as Lixe2x80x94Al, Lixe2x80x94Mg, Lixe2x80x94In, Lixe2x80x94Alxe2x80x94Mn alloys; and metallic Li. In such instances, a charge voltage of about 3.0 V and discharge voltage of about 1.0-1.5 V will be given.
In the case where the aforementioned Lixe2x80x94Ti complex oxide is used as the negative active material, a lithium-containing transition metal oxide, such as LiCoO2, LiNiO2, LiMn2O4, LiMnO2, lithium-containing MnO2, LiCo0.5Ni0.5O2, LiNi0.7Co0.2Mn0.1O2, LiCo0.9Ti0.1O2, LiCo0.5Ni0.4Zr0.1O2 or the like, can be used as the positive active material. In such instances, a charge voltage of about 2.7-3.0 V and a. discharge voltage of about 2.5 V will be given. Those batteries are generally assembled in a discharged state and can be brought to a dischargeable condition by first charging them, i.e., by allowing Li present in the positive active material to migrate into the negative active material. The above-listed positive active materials are under continued study for use in rechargeable lithium batteries requiring relatively large current densities, and their use is more effective in achieving the improvement of load characteristics as contemplated in the present invention.
The Lixe2x80x94Ti complex oxide of the present invention can be synthesized by calcining a mixture of materials containing its constituent elements. The calcining temperature may preferably be within the range of 800xc2x0 C.-950xc2x0 C. If the calcining temperature is below 800xc2x0 C., the production of Lixe2x80x94Ti complex oxide having a spinel structure may become incomplete to result in the failure to obtain a sufficient effect from the introduction of the element M. On the other hand, if the calcining temperature exceeds 950xc2x0 C., the production of a separate phase, such as Li2Ti3O7, may result which could cause the reduction in discharge capacity or in discharge load characteristics.
The electrode active material of the present invention, for use in rechargeable lithium batteries, is characterized as containing an Lixe2x80x94Ti complex oxide represented by the compositional formula Li4MxTi5xe2x88x92xO12 and having a spinel crystal structure, wherein M is selected from at least one of V, Nb, Mo and P, and x satisfies the relationship 0 less than xxe2x89xa60.45.
The use of this electrode active material for rechargeable lithium batteries results in the improvement of discharge characteristics at heavy loads.